7. Analytical Techniques I

Question 1

Describe how molecular ions are formed in mass spectrometry

Answer 1

molecules are bombarded with high energy electrons
this causes the molecule to lose an electron
results in the formation of a positively charged molecular ion with one unpaired electron

Question 2

Why does the M+1 peak occur in mass spectrometry

Answer 2

occurs due to the natural abundance of the carbon-13 isotope in a sample

Question 3

Describe the process of time of flight mass spectrometry (TOF)

Answer 3

1. ionisation - sample is vapourised and bombarded with high energy electrons, forming a positive molecular ion
2. acceleration - ions are accelerated towards negatively charged detection plate
3. ion drift - ions are deflected by a magnetic field
4. detection - ions hit the negatively charged detection plate and gain an electron, producing a flow of charge

Question 4

What is the base peak in mass spectrometry

Answer 4

the peak corresponding to the most abundant ion

Question 5

How does the size and charge of a fragment affect its position on a mass spectrometer

Answer 5

the smaller and more positively charged ions will be detected first as they will get deflected the most and are more attracted to the negative pole of the magnet

Question 6

Why is fragmentation useful in mass spectrometry

Answer 6

can be used to further identify the structure of a compound when many different compounds have the same molecular mass

Question 7

What is characteristic for halogenoalkanes in mass spectrometry

Answer 7

peaks around the molecular ion peak caused by different isotopes of halogens

Question 8

What is characteristic of alcohols in mass spectrometry

Answer 8

tend to lose a water molecule, giving rise to a peak at 18 below the molecular ion peak

Question 9

Give three uses of high resolution mass spectrometry

Answer 9

1. used to confirm molecular formulae
2. provides data accurate to 4 decimal places
3. can be used to deduce between compounds with the same molecular ion peak

Question 10

Describe how infrared spectroscopy works

Answer 10

all organic molecules absorb infrared radiation that causes atoms to vibrate by stretching, bending and twisting

a spectrophotometer irradiates the sample with infrared waves and detects the intensity of the wavelength which goes through the sample

Question 11

What is characteristic of an OH bond in IR spectroscopy

Answer 11

broad absorbance peak

Question 12

What is characteristic in a C=O bond in carbonyls

Answer 12

sharp, strong absorbance peak

Question 13

What is a fingerprint region

Answer 13

occurs between 500 and 1500cm^-1
contains tiny differences between each species which allows for molecules to be identified